btrfs: remove stale comment from btrfs_free_extent()

A comment at btrfs_free_extent() mentions the call to btrfs_pin_extent()
unlocks the pinned mutex, however that mutex is long gone, it was removed
in 2009 by commit 04018de5 ("Btrfs: kill the pinned_mutex"). So just
delete the comment.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -3375,7 +3375,6 @@ int btrfs_free_extent(struct btrfs_trans_handle *trans, struct btrfs_ref *ref)
 	     ref->tree_ref.owning_root == BTRFS_TREE_LOG_OBJECTID) ||
 	    (ref->type == BTRFS_REF_DATA &&
 	     ref->data_ref.owning_root == BTRFS_TREE_LOG_OBJECTID)) {
-		/* unlocks the pinned mutex */
 		btrfs_pin_extent(trans, ref->bytenr, ref->len, 1);
 		ret = 0;
 	} else if (ref->type == BTRFS_REF_METADATA) {